1. Field of the Invention
This invention relates to a radiation sensing device and a Josephson device. More particularly, this invention relates to a device for the detection of nuclear radiation, light energy, etc. using a superconducting tunnel junction, an arithmetic device, and a Josephson device as used for the detection of microwaves.
2. Description of the Prior Art
The radiation sensing device using a superconducting tunnel junction can possibly have more than a ten times greater energy-resolution than that of the conventional high-resolution semiconductor-detectors. The development of this radiation sensing device has been under way in recent years [Applied Physics, Vol. 53, No. 6, pages 532-537 (1984) and A. Barone "Superconductive Particle Detectors," (1988), World Scientific].
The common term "light" actually denotes an electromagnetic wave, similar to the X ray radiation. The photosensor using a superconducting tunnel junction may become highly sensitive to a wide spectrum of light i.e. light having wavelengths from the far-infrared to the ultraviolet spectrum.
Heretofore, for the detection of radiation energy such as X ray radiation and light energy, by the use of a superconducting tunnel junction, a device configured as illustrated in FIG. 23 and FIG. 24 has been developed.
In the sensing device illustrated in FIG. 23 and FIG. 24, a thin insulating layer (tunnel barrier layer) 113 is disposed throughout the entire junction between a superconductor 111 serving as a lower electrode and a superconductor 112 serving as a counter electrode and shaped so as to enclose the upper surface and one lateral surface of the superconductor 111 serving as the lower electrode. In FIG. 24, the reference numeral 127 denotes an upper wiring and the reference numeral 128 a lower wiring.
To improve the performance of the radiation sensing device or the photosensor, the surface area of the superconductor destined to serve as an absorbent for the radiation energy or photon energy must be increased for enhancing the detection efficiency of the impinging radiation or photons. In the device configured as illustrated in FIG. 23 and FIG. 24, such an increase in the surface area entails a problem in that the electric capacitance of the tunnel junction increases and the radiation or photon energy detection signal decreases.
Booth, for the purpose of solving this problem, has proposed a device having a vertical sectional structure as illustrated in FIG. 25 (Applied Physics Letters, Vol. 50, No. 5, pages 293-295 (1987)). Specifically, a superconducting tunnel junction device is produced having a lower electrode side superconductor 111 which is a composite having superposed on the central part of a large surface area of a superconductive layer 116 a superconductive layer 117 having a small surface area and having a smaller energy gap than the superconductive layer 116. An insulating layer 113 is formed on the upper surface of the superconductive layer 117, and superposed on the insulating layer 113 is a superconductor 112 which serves as a counter electrode. This device reduces the electric capacitance and provides for a large detection signal because the insulating layer 113 has a smaller surface area than the superconductive layer 116 serving as the absorbent of the radiation or photon energy. Further, since the superconductive layer 117 having a smaller energy gap than the superconductive layer 116 is joined to the insulating layer 113, the electrons or holes excited in the superconductive layer 116 by the absorption of radiation or photon energy 120 (hereinafter collectively referred to as "excited electrons" for the sake of simplicity) 121, after being diffused inside the superconductive layer 116 and then passed into the superconductive layer 117, are not appreciably returned again to the superconductive layer 116 but are instead contained within the superconductive layer 117. Since the superconductive layer 117 functions as a trapping layer and continues to trap the excited electrons close to the insulating layer 113, the probability that the excited electrons 121 will penetrate the insulating layer 113 by virtue of the tunnel effect and contribute to a single before recombination is improved.
In the device of such a structure as illustrated in FIG. 25 and as proposed by Booth, the efficiency of the collection of the excited electrons in the superconductive layer 117 is high when the mean free path of the excited electrons within the superconductive layer 116 is not very small as compared with the representative length of the plane of the superconductive layer 116 (such as, for example, the diameter where the plane has a circular shape or the length of the side where the plane has a square shape). Generally, however, it is quite difficult to increase the mean free path of excited electrons within a superconductor beyond the level of some tens of .mu.m. Even if the mean free path could be sufficiently increased in a superconductor having a bulky form, once the superconductor is finished in the form of a thin film, the mean free path is substantially limited by the thickness of the thin film. Thus, it is impossible to approximate the mean free path to the representative length of the plane of the superconductive layer 116 in the case of thin film detectors.
In the structure illustrated in FIG. 25, since the surface area of the superconductive layer 117 destined to serve as a trapping layer is smaller than that of the superconductive layer 116 destined to serve as an absorbent of energy, it is inferred that the excited electrons 121 produced as a consequence of the absorption of radiation or photon energy 120 in a portion of the superconductive layer 116 remote from the superconductive layer 117 require a much longer time to reach the superconductive layer 117 by diffusion than those produced in a portion relatively close to the superconductive layer 117 as illustrated in FIG. 26. In fact, it is well known that the average time required for the excited electrons 121 to depart from a certain point to a certain distance by diffusion is proportional to the square of this distance. It is also well known that in a superconductor, the electrons excited by radiation or photon energy eventually recombine (S. B. Kaplan et al, Physical Review B., Vol. 14, pages 4,854-4,873 (1976)). In the tunnel junction device, the excited electrons no longer contribute to detection signals when they recombine before they penetrate the insulating layer by virtue of the tunnel effect. Thus, even when the radiation or photon energy is constant, the rise time and magnitude of the detection signal varies according to the location of the impingement of the radiation or photon energy. Further, concerning the electrons excited in the portion 116 close to the superconductive layer 117, many of the excited electrons diffuse to the portion of the superconductive layer 116 remote from the superconductive layer 117 before they reach the superconductor layer 117.
In the device structure of FIG. 25, when the surface areas of the insulating layer 113 and that of the superconductor 112 are fixed and the surface area of the superconductive layer 117 is enlarged so as to equal that of the superconductive layer 116, the average time required for the electrons excited within the superconductive layer 116 to be diffused in the superconductive layer 117 is decreased. In this case, however, the time required by the excited electrons in traveling from the superconductive layer 117 of a larger surface area to the insulating layer 113 of a smaller surface area is increased and, as the result, the probability that they will recombine before they penetrate the insulating layer 113 by virtue of the tunnel effect increases.
The conventional radiation sensing device is disadvantageous in that a generous increase in the surface area for the enhancement of the efficiency of detection is difficult to obtain, in that a large increase in the surface area results in a decrease in the response speed, and in that the rise time and magnitude of the detection signal varies according to the location of the impingement of the radiation or photon energy.
Further, in the radiation sensing device or the photosensor, it is considered that an increase in the thickness of the superconductor serving as an energy absorbent is effective in enhancing the detection efficiency of radiation or light energy.
It is plain from the above description that in order for the radiation sensing device or the photosensor to be highly sensitive, however, the excited electrons produced in the sensor by the impingement of radiation or photon energy must efficiently pass through the tunnel barrier layer by virtue of the tunnel effect and then output from the junction in the form of signal charges. If the excited electrons require a long time period in tunneling the barrier layer, most of the excited electrons will recombine in this time period and are thus not output in the form of signal charges.
When the thickness of the superconductor is increased for the purpose of improving the detection efficiency of the radiation or light energy and, as the result, the time required by the excited electrons in tunneling the tunnel barrier layer increases, there ensues a disadvantage in that the collection efficiency of the excited electrons is degraded.
In fact, in the conventional radiation sensing device, the energy resolution is seriously degraded by an increase in the thickness of the superconductive layer alone. This phenomenon is ascribable in a large measure to the fact the conventional radiation sensing device configured as illustrated in FIGS. 23 and 24 or FIGS. 25 and 26 utilizes superconductors formed invariably of polycrystals.
Specifically, in order for the excited electrons 121 produced in response to the incident radiation or photon energy 120 to achieve efficient tunneling through the tunnel barrier layer 113, it is desirable that the excited electrons 121 be capable of quickly migrating about within the superconductive layer and easily reaching the tunnel barrier layer. As easily understood from FIG. 27 and FIG. 28 which, depict the manner of diffusion of excited electrons 121, it is desirable that the superconductive layer on which the radiation or photon 120 impinges not be the polycrystalline superconductor 111 but instead be a single-crystal superconductor 131 in which the mean free path of excited electrons 121 is long. Incidentally, this fact has been pointed out in literature (Nuclear Instruments and Method in Physics Research, Vol. 277, P 483 (1984)).
Forming the energy absorbent superconductor of the single-crystal superconductor 131, therefore, is advantageous for enhancing the collection efficiency of the excited electrons. In a device as illustrated in FIG. 28 which is formed using a single-crystal superconductor 131 as a lower superconductive layer and superposing thereon a tunnel barrier layer 113 and an upper superconductive layer 112, there arises a disadvantage in that there is an increase in the leak current, i.e. the electric current which flows through defects in the tunnel barrier layer 131 or in the terminal of the tunnel barrier layer 113. The leak current itself constitutes a major source of electric noise. For the radiation sensing device which measures individual components of radiation or light energy from weak signal charges, this drawback is fatal.
The superconducting tunnel junction type radiation sensing device relies on excited electrons as already described and not on the Josephson effect. The operating principle and the method of use of this radiation sensing device are entirely different from those of the microwave sensor using a Josephson junction, for example. In fact, the radiation sensing device as used is maintained under a magnetic field for the purpose of suppressing the Josephson effect. The wiring for this device also necessitates a peculiar construction which is not required in the Josephson junction. Exclusively in terms of the basic configuration of the junction, however, the superconducting tunnel junction device and the tunnel type Josephson junction are substantially identical to each other.
Heretofore, in the tunnel type Josephson device, polycrystalline superconductors have also been used as superconductors. In a device which uses polycrystalline superconductors only as superconductors, however, when the temperature above the transition temperature is lowered below that temperature in a weak magnetic field, such as in geomagnetism, the superconductor is not easily cooled evenly or uniformly in one direction because of the numerous grain boundaries present in the polycrystalline. Thus, there ensues the possibility that the magnetic field which has been expelled by the Meissner effect from a location already vested with superconductivity is trapped in a location not yet vested with superconductivity and the magnetic flux is consequently trapped in the device (magnetic flux trap). Since the occurrence of the magnetic flux trap depends on a delicate change in the manner of cooling occurring in the device, it cannot be easily controlled.
When magnetic flux is trapped, the Josephson device cannot achieve an optimum operation because the DC Josephson current decreases.
For the elimination of this problem, the use of single crystals as a superconductor layer has been tried in the tunnel type Josephson device (IEEE Transactions on Magnetic, Vol. MAG-21, No. 2, p 539 (1985)).
In the tunnel type Josephson device which uses single crystals as superconductors on the input side, it is considered that the magnetic flux trap does not easily occur because the device is liable to be cooled even or uniformly in one direction and, as the result, the magnetic flux is completely expelled from the device. Where the Josephson device is formed as illustrated in FIG. 28 by using a single-crystal superconductor 131 as a lower superconductive layer and superposing thereon a tunnel barrier layer 113 and an upper superconductive layer 112, there inevitably ensues a disadvantage in that there is an increase in the leak current, i.e. the electric current which flows through the defects in the tunnel barrier layer 113 or in the terminal part of the tunnel barrier layer 113, similar to the aforementioned radiation sensing device. Even in the Josephson device, the leak current poses a serious problem in that the probability of the device producing an erroneous operation will be increased.